1. Field of the Invention
The present invention relates to wireless communication systems, and, in particular, to a technique for determining individual channel propagation characteristics between a multi-element transmit antenna and a multi-element receive antenna in a space-time architecture wireless communication system.
2. Discussion of the Known Art
A so-called layered, space-time architecture for a wireless point-to-point communication system has been recently proposed. See G. J. Foschini, Layered Space-Time Architecture for Wireless Communication in a Fading Environment When Using Multi-Element Antennas, Bell Labs Technical Journal, Autumn 1996, at 41, et seq. This publication ("Foschini") is incorporated by reference herein. See also co-pending U.S. patent applications Ser. No. 08/673,981 filed Jul. 1, 1996; and Ser. No. 09/060,657 filed Apr. 15, 1998, both of which applications are assigned to the assignee of the present application and invention.
The communication system disclosed in Foschini has a potentially high overall bit-rate which can be realized with one-dimensional coding techniques. The system employs M transmit and N receive antenna elements, assumed to be operating in an environment where propagation is substantially frequency-independent over a system transmission frequency band, and with N.gtoreq.M. At discrete times t, a transmitter sends M signals {s.sub.t1, . . . , s.sub.tM }, one from each of the M transmit antenna elements. A receiver records N signals {x.sub.t1, . . . , x.sub.tN }, one from each of its N receive antenna elements, such that: ##EQU1##
where h.sub.mn is a component of an M.times.N complex-valued propagation matrix H that is nearly constant with respect to time, and {v.sub.tn, t=1, . . . ; n=1, . . . N} represents additive receiver noise.
The above expression (Eq. 1) represents all quantities at baseband, and all quantities are complex-valued. The actual real-valued signal that is fed into the m-th transmit antenna element at time t is equal to Real (s.sub.tm).multidot.cos (2.pi.f.sub.o t)-Imag(s.sub.tm).multidot.sin (2.pi.f.sub.o t); where Real ( ) and Imag ( ) denote the real and the imaginary parts of the complex-valued signal, and f.sub.o is a carrier operating frequency of the system.
The Foschini system assumes that the propagation characteristics H are somehow known to the receiver, but not to the transmitter. These characteristics are then used to discriminate among a number of signals transmitted from the transmit to the receive antenna elements over corresponding "layered" communication channels. Signals transmitted over the channels are recovered, for example, by processes called "nulling" and "cancellation". Foschini does not, however, provide specific details as to how the mentioned propagation characteristics are initially determined at the receiver during system operation.
A method of remotely calibrating active phased array antennas on satellites is disclosed in S. D. Silverstein, "Application of Orthogonal Codes to the Calibration of Active Phased Array Antennas for Communication Satellites", IEEE Transactions on Signal Processing, v.45 (January 1997) at 206, et seq. See also U.S. Pat. No. 4,631,734 (Dec. 23, 1986) entitled "Cross-Polarization Canceler/Equalizer".